1. Field of the Invention
The present invention relates to a display device using a light emitting element. Specifically, the present invention relates to an active-matrix type display device including a plurality of pixels that are arranged in matrix where each of the plurality pixels includes a light emitting element. Further, the present invention also relates to an electronic device using the display device.
2. Description of the Related Art
In recent years, a display device using a self-light emitting element has been actively researched and developed. Especially, a display device using a light emitting element typified as an electro luminescence (EL) element has been actively researched and developed for practical application of use of a display.
In the case of displaying a multi-gray scale image by a display device using a light emitting element, a driving method of an analog driving method (an analog gray scale method) or a digital driving method (a digital gray scale method) is employed. The analog driving method is a method in which current magnitude flowing in a light emitting element is continuously controlled to obtain a gray scale. The digital driving method is a method in which a light emitting element is operated by using only two states of an ON state (where the luminance is approximately 100% and the light emitting element emits light) and an OFF state (where the luminance is approximately 0% and the light emitting element emits no light).
As the digital driving method without modification, the light emitting element can display only two gray scales because the light emitting element is operated by using only two states of the ON state and the OFF state. Therefore, a method, which is combined with a driving method displaying a multi-gray scale such as an area gray scale method and a time gray scale method, is employed. The area gray scale method is a method in which a gray scale display is performed depending on the size of a light emitting area of a sub-pixel by providing the sub-pixel in a pixel (for example, see Patent Document 1). Further, the time gray scale method is a method in which a gray scale is displayed by controlling a light-emitting period and frequency of light emission of a pixel (for example, see Patent Document 2 and Patent Document 3).
[Patent Document 1] Japanese Patent Application Laid-Open No. H11-73158
[Patent Document 2] Japanese Patent Application Laid-Open No. 2001-5426
[Patent Document 3] Japanese Patent Application Laid-Open No. 2001-343933
A configuration example of a pixel in a display device employing the digital driving method and operation thereof will be described below. Note that it is difficult to define a source electrode and a drain electrode of a thin film transistor over a substrate having an insulated surface due to a structure thereof. Accordingly, hereinafter, one of a source electrode and a drain electrode is represented as a first electrode, and then, the other of the source electrode and the drain electrode is represented as a second electrode except for a case where a definition of a source electrode and a drain electrode is particularly needed. In general, a lower potential side is a source electrode and a higher potential side is a drain electrode in an N-channel transistor, whereas a higher potential side is a source electrode and a lower potential side is a drain electrode in a P-channel transistor.
A pixel 210 includes a writing transistor 203, a driving transistor 205, and a light emitting element 206 (see FIG. 12). In the writing transistor 203, a gate electrode, a first electrode, and a second electrode are respectively connected to a scanning line 202, a signal line 200, and a gate electrode of the driving transistor 205. In the driving transistor 205, a first electrode and a second electrode are respectively connected to a power source line 201, and a first electrode of the light emitting element 206. A second electrode of the light emitting element 206 is connected to a power source 207. In the light emitting element 206, a structure in which the first electrode is an anode and the second electrode is a cathode, or a structure in which the anode and the cathode are reversed may be employed. In this case, a current direction is changed; therefore, each potential of the power source line 201 and the power source 207 are appropriately determined.
A capacitor 204 is provided to hold voltage between the gate electrode and the source electrode of the driving transistor 205 (hereinafter, referred to as Vgs). The capacitor 204 may be provided between the gate electrode of the driving transistor 205 and the power source line 201; or between the gate electrode of the driving transistor 205 and a wiring that is held at a fixed potential. Alternatively, the capacitor 204 for holding the voltage between the gate electrode and the source electrode of the driving transistor 205 is not provided, and instead, a parasitic capacitance that parasitizes between the gate electrode and the source electrode of the driving transistor 205 may be used in order to hold the Vgs of the driving transistor 205.
The power source line 201 and the power source 207 are held at each predetermined potential, which have a potential difference in each other. The power source line 201 is held at a fixed potential. The second electrode of the light emitting element 206 that is connected to the power source 207 is held at a fixed potential.
When the writing transistor 203 is turned on by a signal inputted through the scanning line 202, a video signal is inputted into the gate electrode of the driving transistor 205 through the signal line 200. A potential difference between the video signal and the power source line 201 is to be the Vgs of the driving transistor 205. When the driving transistor 205 is turned on by value of the Vgs, current is supplied to the light emitting element 206, and the light emitting element 206 emits light. On the other hand, when the driving transistor 205 is turned off by value of the Vgs, current is not supplied to the light emitting element 206, and the light emitting element 206 emits no light. The Vgs of the driving transistor 205 is held by the capacitor 204 for a certain period until a next video signal is inputted into the pixel 210.